1. Field of the Invention
This invention is in the field of integrated operational amplifier devices and methods therefor and, more particularly, is a system for providing a low cost electrically adjustable input offset voltage (.upsilon..sub.os) circuit coupled to an operational amplifier device in a monolithic integrated circuit package.
2. Description of the Related Art
One of the most important characteristics of an operational amplifier device is its input offset voltage. In an ideal operational amplifier, when both input terminals are grounded, the ideal operational amplifier will develop zero output voltage. However, a practical, or real operational amplifier will have some finite amount of voltage at its output when the inputs are grounded. This finite output voltage exists because of the many sources of voltage error inherent within a real operational amplifier device, and collectively the resultant voltage errors are referred to as the equivalent input offset voltage. In order to correct this finite output voltage to a zero amount, or as close as possible, two primary solutions are possible; Either external circuitry must be used to supply an input offset voltage, a solution that adds complexity and circuitry external to the integrated circuit operational amplifier, or the physical structure of the integrated circuit must be adjusted using a laser modification procedure.
The preferred solution to minimize the equivalent input offset voltage has been to trim or adjust the operational amplifier device at the factory to minimize or eliminate the external circuitry required. This trimming of the operational amplifier device is performed at the factory using specially developed equipment to employ a laser modification procedure. This procedure has significant drawbacks economically due to the cost of the laser equipment, and the inability to undo a laser adjustment after the fact.
The present invention allows the processing of bulk numbers of operational amplifiers using extremely inexpensive equipment rather than the very expensive laser method. Additionally, the present invention allows for the re-adjustment of the .upsilon..sub.os circuit, something that is not possible with the present laser .upsilon..sub.os adjustment method. Additionally, the .upsilon..sub.os adjustment may be used to actually set the operational amplifier device to have an output voltage that is zero, positive, or negative for different operating conditions including when the operational amplifier device has both of its inputs grounded. Finally, the present invention will provide for the adjusted input offset voltage value to be retained practically indefinitely by means of a charge storage device embedded within the monolithic structure of the integrated operational amplifier device.
(It should be noted that the terms "equivalent offset voltage" and "input offset voltage" though possessing very slight differences in meaning are generally used interchangeably by those skilled in the art, and the explicit meaning may be contextual based on the usage. Therefor, the use of either "equivalent offset voltage" or "input offset voltage" herein is not meant to be limiting and those well skilled in the art will recognize the interchangeable usage of the terms.)